This invention relates to an upstroke mechanically operated wireline jar for use in the downhole environment of an oil well.
A wireline run in an oil well can perform downhole operations under high pressures and at substantial depths. Pressures of 10,000 psi (6.85.times.10.sup.7 N/m.sup.2) and depths of 15,000 ft (4.572 km) are not uncommon. Solid wirelines known as slicklines, of small diameter and smooth finish, seal and run through stuffing boxes. Commonly found wire thicknesses are 0.092 in. (2.34 mm) and 0.108 in. (2.74 mm) diameter. The wireline is wound onto a winch on the surface. The wireline thickness is as small as possible to minimise the piston effect of the high well pressure below over the atmospheric pressure above, acting on the cross-sectional area of the wireline. The piston effect is kept under control by sinker bars, or weights, at the end of the wire. The wireline diameter is also desirably small to minimise metal fatigue of the wire in use, and for flexibility.
Minimising the wireline diameter has the disadvantage that the force which can be applied to the wireline is limited. To achieve a large force which may be required downhole, a tool known as a jar is used which creates such a force by the impact of one member hammering on another. A simple form of jar, known as a link jar, is operated by pulling the sinker bar up or dropping it down very quickly. This necessitates high speed rotation of the winch, with the possibility of wire fatigue and breakage.
To overcome this problem, prior art jars have been used which comprise an operating rod carrying a hammer and biased against the wireline pull by a spring. After a predetermined tension is achieved, a tripping mechanism operates to release the rod from the action of the spring whereupon the rod flies upwards until the hammer strikes an anvil on the jar casing. In one such device the tripping mechanism and spring are carried on the rod at its bottom end and are subject to damage during the jar stroke. In another such device the spring is situated at the top of the casing and the rod passes through it, the spring force being transmitted to the bottom end of the rod via a prong or yoke arranged around the rod. Such an arrangement limits the length of stroke available for the operating rod. This latter device provides for the adjustment of the spring tension after the removal of a cap at the top of the tool.